All publications mentioned throughout this application are fully incorporated herein by reference, including all references cited therein.
Fibrinogen-like protein 2 (FGL-2)/fibroleukin, also known as FGL-2 prothrombinase has been cloned and identified, and shown to belong to the fibrinogen family of proteins [Ning Q, et al. (2005), J Immunol, 174: 7403-7411]. At the genetic level, mouse FGL-2 (mFGL-2) and human FGL-2 (hFGL-2) have been localized to chromosomes 5 and 7 respectively [Ning et al. (2005) id ibid]. The human gene is approximately 7 kb in length with 2 exons. From the nucleotide sequence of the human gene a 439 amino acid long protein is predicted. The gene encoding FGL-2 was originally cloned from cytotoxic T lymphocytes (CTL) and the encoded protein of 70 KDa shares a 36% homology to the fibrinogen β and γ chains and a 40% homology to the FRED (fibrinogen-related domain) of tenascin [Chan. C W Y et. al. (2003) J Immunol, 170: 4036-4044]. The murine and human proteins share 78% overall identity with greater conservation at the C terminus [Levy, G A et al. (2000) Am J Pathol. 156: 1217-1225; Yuwaraj, S. et al. (2001) Genomics 71: 330-338].
FGL-2 prothrombinase is a transmembrane protein which was shown to have a serine protease activity capable of directly cleaving prothrombin to thrombin in the absence of factor VII or factor X leading to fibrin deposition, thus triggering thrombosis [Ning et al. (2005) Id ibid]. The coagulation activity of FGL-2 was first described in a murine fulminant hepatitis model [Ding, J W et al. (1997) J Virol, 71: 9223-30]. FGL-2 prothrombinase is expressed by activated reticuloendothelial cells (macrophages and endothelial cells) as well by peripheral blood CD4+ and CD8+ T cells, which also secret it, however, the secreted protein is devoid of coagulation activity [Chan et al. (2003) id ibid]. Moreover, it was shown that distinct domains of FGL-2 are responsible for the prothrombinase and immunomodulatory activities of the molecule [Chan et al. (2003) id ibid]. Recombinant FGL-2 protein was previously shown to induce sprouting in vascular endothelial cells [Kim, I. et al. (1999) J. Biol. Chem., 274, 26523-8]. When FGL-2 is expressed as a membrane-associated protein on activated macrophages and endothelial cells, it exhibits a coagulation activity capable of directly cleaving prothrombin to thrombin. FGL-2 accounts for the fibrin deposition and thrombosis associated with both experimental and human allograft rejection, which has been abrogated through the use of FGL-2 neutralizing antibodies or in FGL-2 knock out mice [Ghanekar, A. et al. (2004) J Immunol, 172, 5693-5701: Hancock, W W et. al. (2004) PNAS USA, 101, 3005-3010]. Macrophage induction of FGL-2 occurs through IFNγ, whereas FGL-2 transcription in endothelial cells occurs in response to TNFα, but not IFNγ [Hancock et al. (2004) id ibid]. Experimental data indicate that endothelial cells rather than leukocyte FGL-2 expression accounts for intravascular fibrin deposition [Ning et al. (2005) id ibid].
WO 98/51335 (and its corresponding US 2007/0128198, U.S. Pat. No. 6,642,433 and U.S. Pat. No. 6,403,089) describes the characterization, of the FGL-2 gene from human and mouse origin, as well as the use of antibodies against FGL-2 for the prevention of fibrin deposition associated with endotoxin shock, viral hepatitis, allograft and xenograft rejection, and fetal loss.
Su. K. et al. (2008) describe the expression of human FGL-2 protein and mRNA in tumor tissues [Su, K. et al. (2008) World J. Gastroenterol. 14(39): 5980-5989]. Interestingly, the authors observed that the normal tissue surrounding the tumor did not display overexpression of FGL-2, as observed in the tumor itself.